The present invention relates generally to coherent light generator systems, and more particularly to systems for controlling the frequency of light used in such systems. It is anticipated that a primary application of the present invention will be in telecommunications, but the present invention is also well suited to use in laboratory measurement and other fields.
The ability to measure and control light wavelength or frequency is highly useful in industry and basic research. The telecommunications industry provides one excellent example, and it will be used occasionally herein. [Since the speed of light is constant in a given medium, it should also be understood that the wavelength and the frequency of radiation have a fixed relationship. Thus, although it is possible to speak here of xe2x80x9cwavelength lockingxe2x80x9d or xe2x80x9cfrequency locking,xe2x80x9d for ease of reference xe2x80x9cfrequency lockingxe2x80x9d and variants thereof are used. The term xe2x80x9cwavelengthxe2x80x9d is also used, but sparingly to denote where multiple wavelengths may be present and where multiple channel processing may be desirable.]
Numerous systems exist to measure frequency in some manner, and to control a light source to provide or maintain a specific frequency. These, however, suffer from a number of limitations. Co-pending U.S. application Ser. No. 09/798,721 by one of the present inventors for a xe2x80x9cLight Frequency Locker,xe2x80x9d incorporated herewith by reference, provides a discussion of single channel prior art and of prior art systems known to the present inventors that do not integrate the light source and the light frequency control mechanism, i.e., additional major novel aspects of the present invention. Accordingly, the following discussion provides background particularly germane to the invention of this patent application.
FIG. 1 (background art) is a perspective view of the major operational elements of a single channel frequency locker 10. A laser light source (not shown) produces a laser beam 12 which is directed through a first beam splitter 14 to produce a control beam 16. The laser light source may be quite removed from the locker 10, as implied here, and various optical devices like fiber optic cable may be used to route the laser beam 12 into the first beam splitter 14 or to receive it as it exits and route it onward for use in some end application. Typically only a small portion of the laser beam 12 is xe2x80x9csplitxe2x80x9d out in this manner and used as the control beam 16.
The laser beam 12 has a single light wavelength of interest, although others may also be present so long as they do not significantly effect the operation of the locker 10. In particular, in many applications the laser beam 12 is modulated to carry information. The purpose of the locker 10 is to lock the light frequency of the laser beam 12 to a desired frequency.
The control beam 16 is directed into a second beam splitter 18 to produce a reference beam 20 and a measurement beam 22. The reference beam 20 is directed to a reference detector 24, where it produces a reference signal 26. The measurement beam 22 is directed to an interferometer 28 to produce an interference beam 30. The interference beam 30 is then directed to an interference detector 32, where it produces an interference signal 34.
The beam splitters 14, 18, the detectors 24, 32, and the interferometer 28 may be conventional commercially available units. For example, the beam splitters 14, 18 may be what are often termed xe2x80x9chalf-silvered mirrors,xe2x80x9d although the reflective material may not be silver and the reflectivity to transmitting balance may not be half and half. The detectors 24, 32 may be photodetectors, such as photodiodes.
Many types of devices are suitable for the interferometer 28. An air-spaced Fabry-Perot etalon is shown in FIG. 1, but solid etalons or diffraction gratings are examples of other suitable devices. In particular, however, the interferometer 28 is chosen to produce a usable amount of interference for the desired frequency of the laser beam 12.
A processing circuit 36 is further provided to receive both the reference signal 26 and the interference signal 34 and to produce a correction signal 38. The reference signal 26 is representative of the xe2x80x9crawxe2x80x9d light intensity in the control beam 16 at any given moment. In contrast, the interference signal 34 is representative of the light frequency in the control beam 16, and thus also in the laser beam 12. By combining these signals, typically using differential amplification techniques, the processing circuit 36 is able to normalize for intensity variation in the control beam 16 and to further determine if frequency variation, i.e. xe2x80x9cdrift,xe2x80x9d has occurred. It then can produce the correction signal 38 accordingly.
The correction signal 38 is used as feedback to the laser light source to achieve frequency control as the laser beam 12 is being produced. In this manner any drift can be detected while it is still minor and can promptly be corrected for, thus xe2x80x9clockingxe2x80x9d the light frequency to the desired frequency. The above discussion of the single channel frequency locker 10 is brief and does not cover non-germane matters, like tuning to an initial light frequency, but rather is intended to serve as a basis for the following discussion.
The locker 10 shown in FIG. 1 illustrates several points. It is a stand alone unit, physically separated from the laser light source it is used with. Historically this has been the case in this art. Laser modules, containing a laser and a mechanism to control its light frequency, have been produced as one physical unit while the frequency lockers that direct operation of the control mechanism have been separate physical units. The laser modules and locker units are then combined, typically by a designer for use in an end application.
This unfortunately has a number of disadvantages. The costs of this approach are unduly high. There is an added direct cost for using two different units, often from two different providers. Another consideration is the added indirect cost of designing combinations into end applications where there ultimately is only one problem to be solved: providing a frequency locked light source.
Of growing importance, also, is the ultimate form-factor of a frequency locked light source. Using two discrete parts tends to undesirably increase the surface area or volume required. Today minimizing the form-factor is important in many applications, particularly as many such applications use multiple frequency locked light sources together and the surface area and volume required for this becomes quite appreciable.
There are yet other disadvantages, such as minimizing counts of stocked spares, the economics of dealing with multiple vendors, and even operational interference between the units. For instance, if a laser module uses heating to adjust light frequency, waste heat from this can adversely effect the frequency locker unit. Similarly, if cooling is used, adjacent components may be cooled somewhat as well. It is not possible to catalog all of the possible disadvantages here, but those noted are major ones and they serve to make the point that using two devices to solve one problem may be unduly complex or expensive.
FIG. 2 (background art) is a schematic view of a single channel frequency locked light source 50, essentially including the single channel frequency locker 10 of FIG. 1. A laser module 54 is now provided. It includes a laser chip 56 able to produce the laser beam 12, with some collimating optics 58 also depicted.
The laser module 54 here includes a temperature unit 60. Heating and cooling the laser chip 56 are common ways to adjust the frequency of the laser beam 12. The temperature unit 60 here may do either, or even both across time as operating requirements change.
A control circuit 62 that is somewhat different than the processing circuit 36 of FIG. 1 is shown, and the correction signal 38 is now one among other control signals 64. The control circuit 62 here includes a differential amplifier 66. As noted, above, a differential processing approach is commonly used to normalize with respect to light intensity variation in the laser beam 12. The control circuit 62 also includes a processing circuit 68 that performs both frequency stabilization and modulation related processing here. There is much to be gained by integrating all of the processing tasks into one circuit in the manner of the control circuit 62 here. For instance, a common power supply and micro-processor components can be used in the processing circuit 68.
Those skilled in the relevant arts here, and particularly in the optical and electronic arts, will appreciate that combining light source and frequency locking into one unit like the locked light source 50 can overcome or reduce many of the disadvantages for multiple discrete devices discussed above. It is the inventors"" understanding that integrated single channel frequency locked light sources like that just described are just now entering the market.
FIG. 3 (background art) is a schematic view of a multi-channel (i.e., typically multi-wavelength) locked light source 80. This represents the known current state of the art when it comes to producing multiple frequency locked laser beams 82. Essentially, a plurality of the locked light sources 50 of FIG. 2 are used to construct an aggregate unit 84. The burden of doing this, however, is still left for the designer of the end application.
The locked light source 80 in FIG. 3 is a three channel system, able to produce three of the laser beams 82 that each may have a respective light frequency. This is actually a quite simple system, in view of current end application complexity.
An appreciation for the cost and effort involved in designing end applications around the currently available technology can be had by considering the increasingly common example of dense-wavelength division multiplexing used in fiber optic telecommunications. Today 40 and even 80 channel fiber optic systems are in use, and production of 160 and 320 channel systems are contemplated. A corresponding number of modulated light wavelengths is needed, and these need to be frequency locked, say, to the standard frequencies set by the International Telecommunication Union (ITU). Employing even 40 discrete instances (i.e., channels) of the locked light sources 50 of FIG. 2 is daunting, however. If the frequency of a single laser (i.e., channel) drifts it can adversely effect both its own utility as well as the utility of an adjacent channel. This can undermine the reliability of and the confidence in the entire system. Furthermore, it is even likely that frequency drift in such systems will occur concurrently in multiple channels; adjacent channels can then drift together and interfere even more easily.
Turning again to FIGS. 2 and 3, it can be seen that the locked light source 80 includes a lot of component similarity. For example, each locked light source 50 or xe2x80x9cchannelxe2x80x9d there has a first beam splitter 14, a second beam splitter 18, an interferometer 28, a differential amplifier 66, and a processing circuit 68. Each such similar component has an associated cost by itself, as it relates to its role within the channel, and within the overall scheme. Each optical and electrical component must be individually fabricated, tested, installed, and adjusted. Yet, if merely one fails, the entire locked light source 80 may be rendered unusable or deemed untrustworthy. The disadvantages of such an approach are many.
Accordingly, it is an object of the present invention to provide an improved frequency locker.
Another object of the invention is to provide an improved frequency locking light source.
And another object of the invention is to provide multi-channel frequency locking light source.
Briefly, a second preferred embodiment of the present invention is a multi-channel frequency locker to lock a number of tunable light sources producing light beams having respective light frequencies. A detection system is supplied to produce and provide to the tunable light sources respective correction signals representing any differences in frequency between the light frequencies and desired frequencies. The detection system includes at least one optical component common to the light beams.
Briefly, a second preferred embodiment of the present invention is a frequency locking light source. A laser is provided to produce at least one light beam having a light frequency, and a tuning system is provided to adjust the laser to change the light frequency based on a correction signal. A detection system is provided to produce the correction signal representing any difference in frequency between the light frequency and a desired frequency. The laser, tuning system, and detection system are all integral within a single housing.
Briefly, a third preferred embodiment of the present invention is a multi-channel frequency locker to lock a number of tunable light sources producing light beams having respective light frequencies. A detection system is supplied to produce and provide to the tunable light sources respective correction signals representing any differences in frequency between the light frequencies and desired frequencies. The detection system includes at least one optical component common to the light beams.
An advantage of the present invention is that it provides a frequency locker which may handle multiple wavelength-channels without a per channel scaling of components or manufacturing and usage upkeep related labor.
Another advantage of the invention is that it may be integrated with a light source and tuning mechanism, or multiple channels of these, to form complete frequency locking light source solutions.
Another advantage of the invention is that coincides with the growing need in industry for small form-factor frequency locking capability, both for single and multi-channel applications.
Another advantage of the invention is that its multi-channel embodiments inherently have easy initial calibration capability, easy recalibration capability once in use, and improved reliability.
And another advantage of the invention is that is it highly economical, both to fabricate and to operate. The invention""s use of common and integrated components reduces materials costs significantly, particularly since the very components that the invention can combine are those typically costing most in prior art approaches. The invention similarly requires less labor in assembly and set-up. It provides a complete locked light source, if desired, and thus avoids the prior art design burden of having to work with tunable light sources and tuning and stabilization systems that are discrete.